1. Field of the Invention
The invention pertains to printers. More particularly, the invention is directed to a printer that can not only provide satisfactory printing conditions as quickly as possible but also reduce noise by improving the sheet discharge structure. The invention is further directed to a printer that can provide stable sheet discharge operation with a simple structure for a type of printer in which one of a pair of sheet discharge rollers is mounted on the printer cover.
2. Related Art
FIG. 5 is a schematic side view showing a printer disclosed as related art in Japanese Unexamined Patent Publication No. 3-53961.
In FIG. 5 reference numeral 1 denotes a case of a printer. Within the case 1 are a platen 2 with a sheet P wrapped therearound, a print head H disposed so as to confront the platen 2, and a pair of sheet discharge rollers 3 that discharges the sheet P printed by the print head H.
The pair of sheet discharge rollers 3 include: a drive roller 3a, which is a first sheet discharge roller; and a driven roller 3b, which is a second sheet discharge roller.
The drive roller 3a is mounted on the case main body 1. The driven roller 3b is mounted on a cover 5.
The drive roller 3a receives motive power from the platen 2 and is driven at an increased circumferential speed with respect to the circumferential speed of the platen 2. The driven roller 3b rotates while abutted against the drive roller 3a.
The case 1 has a sheet discharge opening 4. The cover 5 is fixed so as to be turned around a shaft 5b.
According to the thus constructed printer, the sheet P wrapped around the platen 2 and printed by the print head H is forwarded by the pair of sheet discharge rollers 3 and discharged outside the case 1 from the sheet discharge opening 4.
Since the sheet discharge roller 3b, which is one of the pair of sheet discharge rollers 3, is mounted on the cover 5 that can be opened and closed, when the sheet P is jammed at the sheet discharge roller section, the jammed sheet can be removed with ease by opening the cover 5.
However, the above-mentioned printer has at least the following three problems.
Problem 1
In the above-mentioned printer generally the printing sound produced by the print head H is the major source of noise. Particularly, if the print head is of a wire dot type, the sound of the wire ends impacting the sheet aggravates the noise. On the platen 2 side, such noise is absorbed by the platen 2 itself, so the problem of this noise exists mainly on the print head H side.
In addition to the noise directly produced by the print head H, a noise caused by vibrations of the sheet itself that is vibrated by the print head H must also be taken care of.
In the above-mentioned conventional printer, there is a gap between the sheet P that is in the course of being discharged and an edge portion 4a on the print head H side of the sheet discharge opening 4. A noise leaking from this gap has been the problem.
To overcome this problem, printers have been disclosed in U.S. Pat. No. 4,943,173, and Japanese Unexamined Patent Publications Nos. 1-97674 and 60-17186. These printers are characterized as arranging a sound absorbing material at the sheet discharge opening and thereby causing a sheet to be abutted against the sound absorbing material.
However, such structure characterizing these printers requires additional sound absorbing members arranged at the sheet discharge opening of the printer case, which not only increases the number of parts but also makes the sheet discharge opening structure complicated.
On the other hand, a drawing in which a sheet is abutted against the edge portion of a case at the sheet discharge section is disclosed in Japanese Unexamined Patent Publication No. 2-14172.
However, this printer does not have a guide that allows the sheet to come in slidable contact with the edge portion of the case. As a result, it is not likely that the sheet will come in slidable contact with the edge portion of the case with a desired degree of certainty, thus not ensuring noise reduction.
Problem 2
To ensure satisfactory sheet discharge operation by the pair of sheet discharge rollers 3 in a printer such as shown in FIG. 5, it is preferable that both the drive roller 3a and the driven roller 3b be made of a material whose friction coefficient with respect to paper is larger, e.g., of rubber.
However, if the driven roller 3b is made of rubber to which ink is easy to adhere, the ink on a print surface Pa of the sheet P adheres to the driven roller. Then, the ink on the driven roller is further transferred to the print surface Pa of the sheet P, thereby contaminating the print surface, which is a problem.
This problem can be overcome by making the driven roller 3b of a material to which ink is hard to adhere, e.g., of synthetic resin.
However, if the driven roller 3b is made of synthetic resin, the friction coefficient of the synthetic resin with respect to paper is smaller than that of rubber, which imposes another problem.
That is, when the leading end of the sheet printed by the print head H is guided so as to be abutted against the driven roller made of synthetic resin, it takes time for the leading end of the sheet to be guided to the drive roller 3a and nipped between the pair of sheet discharge rollers 3 so as to have a forwarding force imparted thereto because the friction coefficient of the surface of the driven roller with respect to the sheet is small.
The sheet P is not wrapped around the platen 2 in contact with the platen 2, but floats up from the platen 2 surface until the forwarding force is given upon the leading end of the sheet having been nipped between the pair of sheet discharge rollers 3. As a result, satisfactory printing conditions are not always ensured. Particularly, if the print head is of the wire dot type, vibrations of the sheet caused by the wire ends colliding with the sheet are increased, which in turn causes noise.
Problem 3
Since the second sheet discharge roller (driven roller) 3b is disposed at a position remote from the cover 5 fixing section 5b of the cover 5 in a printer such as shown in FIG. 5, positioning accuracy of the second sheet discharge roller (driven roller) 3b to the first sheet discharge roller (drive roller) 3a is not satisfactory, thereby not ensuring stable sheet discharge operation, which is still another problem.
To overcome this problem, a structure characterized as arranging a member on the case main body side for positioning the shaft of the driven roller is disclosed in Japanese Unexamined Utility Model Publication No. 4-560.
However, this structure is problematical in that a positioning member other than the cover is required and this makes the structure complicated.